Most gas cleaning systems of blast furnace plants comprise a preliminary cleaning stage and a fine cleaning stage. The fine cleaning stage generally consists of at least one gas scrubber or electrostatic separator. The object of the preliminary cleaning stage is to remove coarse (large size) dust particles from the gas flow, before the latter enters the fine cleaning stage. Hereby, the preliminary cleaning stage not only increases separation efficiency of the fine cleaning stage, but also makes its operation more reliable and economical. In most blast furnace plants built before the year 2000, the preliminary cleaning stage consists of a so-called dust-catcher.
Such a dust-catcher (see FIG. 1) is a large vertical pressure vessel comprising top-down: a gas inlet and outlet dome, a big cylindrical separation chamber and a funnel-shaped dust hopper. The gas inlet and outlet dome has a gas inlet connection centred on the vertical axis of pressure vessel and a lateral gas outlet connection near the top end of the dome. The axial gas inlet connection is formed by the upper end of a diffuser pipe, which extends axially through the dome into the separation chamber, where it forms an outlet opening in an horizontal plane. The lateral gas outlet connection opens into an annular space surrounding the diffuser pipe within the dome of the dust-catcher.
Operation of such a dust-catcher is as follows. The raw blast furnace gas (i.e. heavily dust charged gas coming from the blast furnace) descends through a large diameter gas pipe, called “down-corner”, which is connected from above to the gas inlet connection of the dust-catcher. Through the diffuser pipe, the raw gas flow is axially introduced into the separation chamber of the dust-catcher. The increase in cross-section on entry of the gas into the separation chamber results in a considerable reduction of its velocity. In the separation chamber, the gas flow has furthermore to reverse its flow direction from downward to upward, because the sole gas outlet connection of the dust-catcher is located in the dome of the latter. Due to the reduction of velocity and the 180° reversal of flow direction in the separator chamber, the coarsest dust particles fall out by gravity and are collected in the funnel-shaped dust hopper. From here, dust is discharged out of the dust-catcher through a dust discharging lock connected to the bottom end of the funnel-shaped dust hopper. The pre-cleaned gas flow leaves the dust-catcher through the lateral gas outlet connection in the dome.
A conventional dust-catcher as described hereinbefore has a separation efficiency of only 40-50% of the total dust content. This rather poor separation efficiency is not sufficient for an efficient and economical operation of a fine cleaning stage complying with present environmental protection requirements. In modern gas cleaning systems of new blast furnace plants, the preliminary cleaning stage is therefore most often a tangential or an axial cyclone, which have a fare better separation efficiency than a dust-catcher.
An axial cyclone dust separator for a blast furnace gas cleaning system is disclosed in WO 00/40763. This modern dust separator comprises a vertical pressure vessel with an axial feed device for the blast furnace gas at its top end. This axial feed device includes two upward projecting inlet connection pipes for the raw blast furnace gas and a central outlet connection pipe for the pre-cleaned gas. The two inlet connection pipes open into the dome of the pressure vessel, laterally of the central outlet connection pipe. A downward expanding inlet bell is arranged below the dome of the pressure vessel an guides the gas flow towards an annular gap formed between the bottom edge of the inlet bell and the outer wall of the pressure vessel. A swirling device with guide vanes, which is arranged in this annular gap, causes the blast furnace gas to swirl about the vertical axis of the pressurized vessel. Centrifugal forces project the dust particles radially outward, where they collide with the outer wall and then slide downward through a funnel into a dust hopper. The gas flow reverses its downward spiral on a deflection cone located in front of the outlet opening of the aforementioned funnel and moves upward in a smaller inner spiral. The central outlet connection pipe axially traverses the inlet bell and has its inlet opening located below the latter. Through this inlet opening, the upward moving gas flow enters into the central outlet connection pipe through which it axially leaves the dust separator. It will be appreciated that an axial cyclone dust separator of this type achieves a separation efficiency of up to 85% of the total dust content.
Such an axial cyclone separator is consequently a very interesting solution for new blast furnace gas cleaning plants. If one has to revamp an existing blast furnace gas cleaning plant including a conventional dust-catcher, WO 00/40763 suggests to insert an axial cyclone separator without dust hopper into the truncated pressure vessel of the dust catcher, such that the dust hopper is formed by the pressure vessel of the dust catcher. This solution is of course less expensive than completely replacing the conventional dust-catcher by an entirely new axial cyclone separator, but is sometimes still too expensive, if the client has only a limited budget for environmental protection measures.
In case of a revamping of an existing blast furnace gas cleaning plant including a conventional dust-catcher, it would be particularly advantageous to increase the separation efficiency of the conventional dust-catcher without modifications to its pressure vessel. It will indeed be appreciated in this context, that modifications to the pressure vessel are particularly costly, because they require a new pressure certification of the latter.
A solution for increasing the separation efficiency of an existing dust-catcher without modifications to its pressure vessel is disclosed in EP 1557218. This solution consists in mounting several relatively small tangential cyclones within the unmodified pressure vessel of a conventional dust-catcher. Each of these small tangential cyclones consists of an outer tube, a coaxial inner tube and a tangential feed pipe. The latter penetrates at the top end of the outer tube into the annular space delimited between the outer and inner tube. The lower end of the axial diffuser pipe of the dust-catcher is equipped with a central distribution chamber around which the small tangential cyclones are arranged. The blast furnace gas flows through the axial diffuser pipe into this distribution chamber and enters through the tangential feed pipes into the cyclones. Here, the gas is subjected to a spiral downward movement. Dust particles in the gas, which are subjected to centrifugal forces in this spiral downward movement, separate from the gas flow and discharge through the open base of the cyclone into the funnel-shaped end of the dust-catcher. The pre-cleaned gas flows through the inner tubes of the cyclones into the space surrounding the axial diffuser pipe and leaves the dust-catcher through the outlet connection of its pressure vessel. A drawback of the solution disclosed in EP 1557218 is e.g. that dust may also accumulate in the space surrounding the axial diffuser pipe, which has—with the exception of the inner tubes of the cyclones—no communication with the dust hopper of the dust-catcher.